Apparatus for separating oxygen from ambient air

ABSTRACT

An apparatus for separation of gases from ambient air that has at least one separation column with an inlet at a first end and an outlet at a second end, a buffer column having a single inlet at a first end, a vacuum pump, and a valve system that connects the vacuum pump to the outlet at the first end of the separation column, and that connects the outlet at the second end of separation column to the single inlet at the first end of the buffer column.

CROSS-REFERENCE TO RELATED APPLICATION

Reference is made to the following pending applications: U.S. patentapplication Ser. No. 12/839,979 entitled “METHOD OF SEPARATING ANDDISTRIBUTING OXYGEN” as filed on even date herewith, and claims priorityto U.S. Provisional Pat. App. Ser. No. 61/227,545, entitled “APPARATUSFOR SEPARATING OXYGEN FROM AMBIENT AIR” filed Jul. 22, 2009, which ishereby incorporated by reference in its entirety.

BACKGROUND

The present disclosure relates to an adsorption based gas separationapparatus that separates oxygen from ambient air with beneficialimprovements over the prior art in size, weight and energy consumed perunit of oxygen separated from air.

Pressure swing separation systems are known to offer the best energyefficiency for oxygen separation at small production rates. Systemsusing the Skarstrom PSA cycle (described in Pressure Swing Adsorption byDouglas M. Ruthven et al., John Wiley & Sons, Inc., 1994, hereinbyincorporated by reference, Section 3.2.1) for medical oxygen productionare common examples of these small scale systems and are familiar tothose skilled in the art. These systems are known to consume about 350watts of electrical power to produce about 5 liters per minute of oxygenwith about 90% purity.

The portable concentrators are battery powered to eliminate tetheringutility cords and similar power supply structures attached to theapparatus. Research efforts were directed toward energy efficiency ofthe separation process. As previously noted, pressure swing separationsystems are known to offer the best energy efficiency for oxygenseparation at small production rates. Prior art oxygen separators weremeasured, and the performance and behavior of one of these commonly usedsystems was analyzed—the currently popular Invacare model IRC5LX(specifications available from www.invacare.com). In this device, anelectrically driven pneumatic compressor cycled air from 1 to 3atmospheres pressure, following the Skarstrom cycle steps, through twocolumnar containers of adsorbent having high adsorption capacity fornitrogen and other polar molecules. By measuring the pneumatic power ofthe pumped gas stream including the power associated with the adiabaticheating of the pumped gas and comparing that to the electrical powerinput to the pump, it was determined that much energy was being lost topump inefficiency. This finding was set aside, and research was focusedon exploring the power consumption aspects of the separation cyclesteps.

There is a specific amount of theoretical power associated with pumpinga stream of gas through a given pressure difference as is required tooperate a pressure swing separation system. For example: (1Liters/Minute flow rate)×(1 Atmosphere pressure rise)=0.592 watts ofpower. Adiabatic heating of this gas stream consumes additional power inwatts. Reducing the pressure change or the flow rate of the pumped gasstream needed to drive the separation cycle steps would thereforedirectly reduce the power required to produce a given amount of oxygen.Research and testing was done to determine the minimum pump pressure andflow rates to produce a given oxygen separation rate.

SUMMARY

An apparatus for separation of gases from ambient air is disclosed. Theapparatus has at least one separation column with an inlet at a firstend and an outlet at a second end, a buffer column having a single inletat a first end, a vacuum pump, and a valve system that connects thevacuum pump to the inlet at the first end of the separation column, andthat connects the outlet at the second end of the separation column tothe single inlet at the first end of the buffer column.

In a second embodiment, a method of using a gas separator to separateambient air to obtain a stream of gas containing at least 86% oxygen isdisclosed. The method has the steps of: 1) feeding adjacent ambient airat about 1 atm into a first end of a separation column filled with theadsorbent; 2) drawing oxygen rich gas from a second end of the column;3) evacuating waste gas from the first end of the column through avacuum pump to a pressure of less than 1 atm; and 4) repeating steps 1through 3. The separator containing less than 500 cc of an adsorbentwith an isotherm defining its nitrogen capacity as a function ofnitrogen partial pressure to selectively remove nitrogen from ambientair that results in oxygen rich air being produced.

In another embodiment, an apparatus for separation of gases from ambientair has at least one separation column, the separation column containingan inlet at a first end and an outlet at a second end, wherein thecolumn comprises an adsorbent having a high capacity for nitrogenadsorption. The apparatus also has buffer column having a single inletat a first end, wherein the buffer column comprises an adsorbent havinga high capacity for nitrogen adsorption, and a valve system thatconnects the separation column and buffer column.

In another embodiment, a method of separating ambient air to obtain astream of oxygen rich gas is disclosed. A separation column is evacuatedto a first pressure through a feed end of a column, and a flow of gas ismetered from a last in/first out (LIFO) buffer column into a product endof the separation column while continuing to evacuate the column to asecond pressure. Evacuation of the column is stopped while continuingthe flow of gas from the LIFO buffer column at a second flow rate untila third pressure is reached in the separation column. The separationcolumn is pressurized with ambient air through the feed end to apressure of about 1 atm, and air is fed into the feed end of the columnwhile drawing an oxygen rich gas from the product end of the separationcolumn. These steps are then repeated.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of an oxygen separator utilizing a singleadsorption column.

FIG. 2 is a graph of the isotherms of two adsorbents.

FIG. 3 is a schematic view of an oxygen separator utilizing threeidentical adsorption columns.

FIG. 4 is a schematic view of an oxygen separator utilizing twoadsorption columns with a last in/first out buffer column.

DETAILED DESCRIPTION

The separation apparatus described herein was developed for use in anoxygen concentrator that provides portable supplementary oxygen topatients with lowered blood oxygen levels. The concentrator is muchlighter, smaller and more energy efficient than prior art devices, andis small enough so that the apparatus can be conveniently worn by apatient as they move about rather than wheeled on a cart or carried likea piece of luggage. The separation components contributed directly tothe unique portability features of the concentrator. The apparatus canbe scaled in size for higher or lower oxygen separation rates using theunique design features to provide reduced size, weight and powerbenefits for other applications. Configurations that used one, two andthree adsorption columns all using the same separation cycle steps werebuilt and tested.

Research began by exploring the minimum pump pressure and flow rateswith a simple form of pressure swing separation system. The physicalmodel in FIG. 1 was constructed and connected instrumentation toaccurately measure gas pressures, flow rates and compositions as itcycled through a sequence of separation steps. In FIG. 1, 100 is a 1inch (2.54 cm) diameter by 4 inch (10.16 cm) long cylinder (or column)packed with adsorbent beads. Ambient air at approximately 1 atmosphere(101 kPa) can flow into 100 at the feed end adjacent monitoring point101 by way of solenoid valve 110 from port 120. Waste gas can be pumpedfrom 100 at feed end 101 through valve 111 by vacuum pump 150 and ventedto atmosphere through port 121. A gas stream reduced in nitrogen can bepumped from 100 at product end 102 through check valve 112 by pump 151and transferred to accumulator 160. The components in boxes 200 and 250were not present during these first tests. Monitoring points 101 through105 are locales where instrumentation was attached to monitor pressure,flow, and composition of the gas streams as the gas streams variedduring cycle steps. Customized instrumentation was developed that had apressure response time of less than 1 millisecond, a flow rate responsetime of 10 milliseconds, and a composition response time of less than750 milliseconds. The customized instrumentation fed a digital storageoscilloscope that could capture time variant measures of the parameters.The instrumentation proved crucial to measuring and understanding thedetailed behavior of the experimental configurations.

The adsorbent materials useful for oxygen separation have an adsorptioncapacity for nitrogen that is higher than the adsorption capacity foroxygen over a pressure range of several atmospheres. These adsorbentmaterials are characterized by their isotherms. Isotherms are graphsshowing the amount of gas contained in a given mass of adsorbentmaterial as a function of pressure at a fixed temperature. The isothermsfor nitrogen and oxygen are of primary interest. Air is nearly 1% argon.The argon isotherms for these adsorbents are nearly identical to theoxygen isotherms so the separation steps discussed here relate to oxygenand argon as though they were the same. These isotherms for twodifferent adsorbents are shown in FIG. 2.

The type 13X adsorbent is commonly used in small oxygen concentratorssuch as the Invacare device that was tested. Another available adsorbenttype is LiLSX. Isotherms of LiLSX are plotted in FIG. 2 with the 13Xisotherms for comparison. As shown in FIG. 2, the ratio of nitrogencapacity to oxygen capacity is much higher for the LiLSX than the 13X at2 atmospheres pressure and below. The difference of the adsorbents iseven more pronounced at 1 atmosphere and below where the absolutenitrogen capacity of the LiLSX is seen to be much higher than the 13Xwhile the oxygen capacities are similar between the two.

Thus, it was determined that using the LiLSX adsorbent could provideenergy savings over adsorbents such as 13X with lesser nitrogen tooxygen capacity ratios. Any pressure driven oxygen separation sequencefor separating oxygen from air must include the minimum steps ofincreasing the pressure of an air stream while passing it through theadsorbent to adsorb nitrogen, followed by decreasing the pressure todesorb the nitrogen from the adsorbent and carry it away in a waste gasstream. The amount of oxygen that can be separated from the air streamin one cycle increases with increased amounts of nitrogen that can beadsorbed from that air stream. The nitrogen isotherms indicate theamount of nitrogen that is adsorbed for a given pressure change. Thehigher the slope of the nitrogen isotherm curve, the less pressurechange is required to change the amount of nitrogen adsorbed by a givenmass of adsorbent. In FIG. 2, to change the amount of nitrogen held in amass of 13X adsorbent by the amount N1, a pressure change P1 isrequired. With the LiLSX adsorbent, an amount of nitrogen N2 in thefigure required pressure change P2. N2 is approximately equal to N1, butP2 is only about ⅓ the pressure change of P1.

In a portable concentrator, pump 151 is providing the pneumatic power toproduce this pressure change. As described, pressure difference acrossthe pump linearly affects the pumping power input. Operating over apressure range that spans a steep segment of the nitrogen isotherm couldallow a given volume of nitrogen to be cyclically adsorbed and desorbedwith relatively less power provided by the pump. In FIG. 2, the oxygenisotherms for the two adsorbents are more similar but the 13X oxygenisotherm shows much more oxygen being adsorbed over the larger P1pressure range than the LiLSX oxygen isotherm over the smaller P2pressure range. More oxygen will be adsorbed during the high pressurenitrogen adsorbing steps using the 13X adsorbent than in the lowpressure steps using the LiLSX adsorbent. The adsorbed oxygen subtractsdirectly from the amount of product oxygen that can be produced. Usingthese two effects in combination, more oxygen should be produced fromthe same amount of nitrogen pumped over a smaller pressure range in asystem using the LiLSX adsorbent than from one using 13X. UtilizingLiLSX adsorbent in a portable concentrator system will greatly reducethe pneumatic energy required to separate a given volume of oxygen.

The behavior of the LiLSX adsorbent (OXYSIV MDX from UOP LLC) wasexplored in the steep section of its nitrogen isotherm in pressureranges from 1 atmosphere (101 kPa) and below using the simplified testsystem of FIG. 1 without blocks 200 or 250. The system is commonlyreferred to as a vacuum swing type system. The initial cycle stepsconsidered were:

1. Evacuate the column 100 through the feed end adjacent point 101 to avacuum level V measured by a pressure sensor at point 102.

2. Pressurize the column to 1 atmosphere with air through the feed end.

3. Feed air through the feed end at near 1 atmosphere pressure at aconstant flow rate while pumping oxygen rich gas out the product end 102into the accumulator 160.

The process is similar to the vacuum swing cycle from Ruthven et al.,Section 3.2.4. An abrupt increase in the gas flow rate from the productend of the column measured by a flow sensor at point 103 during step 3marked the completion of nitrogen adsorption in the column and the endof step 3. At that point in time, the entire column was at equilibriumwith atmospheric nitrogen partial pressure of about 0.78 atmospheres (79kPa) and oxygen (including about 1% argon) at 0.22 atmospheres (22 kPa).The cycle repeats with step 1 resetting the nitrogen equilibrium in thecolumn to a lower level according to the end vacuum level V. The lowoxygen partial pressure throughout the entire column at the end of step3 left very little oxygen to be lost to the waste stream during step 1.

By measuring the performance of this test system, how vacuum level Vaffected the ratio of power consumed to oxygen produced by this cyclewas determined. Rather than considering the oxygen produced, the systemwas viewed as nitrogen eliminated from the product stream. As the vacuumlevel V in step 1 was decreased (absolute pressure lowered) the amountof nitrogen removed from the product stream during step 3 increased atan increasing rate for each additional decrement of V as predicted bythe nonlinear increasing slope of the nitrogen isotherm. Oxygenadsorption also occurred according to the oxygen isotherm. Oxygen wasthen lost in the waste stream during step 1 as it was desorbed duringthe evacuation of the column. However, the oxygen isotherm is nearlylinear so there was a linear increase in the amount of lost oxygen withdecreased V while the amount of nitrogen removed increased at anincreasing rate. Stronger end vacuums therefore increased oxygenproduced, due to larger volumes of adsorbed nitrogen, at an increasingrate.

Another important performance factor is the purity of the product oxygenstream and how it was affected by V. Industry standards required adesign to produce oxygen with a purity of 86% or higher. The productpurity increased with reduced V (stronger vacuum). Thus, the evacuatestep 1 had to reduce the nitrogen partial pressure in the column to lessthan 0.08 atmosphere (8.1 kPa) to produce the necessary oxygen purityfrom the feed step 3 that would follow.

Strong vacuums, including those below 0.1 atmosphere (10 kPa) presentproblems when considering the practical characteristics of vacuum pumps.The volumetric efficiency of positive displacement vacuum pumpsdecreases in proportion to the vacuum level being pumped. For example,at 0.1 atmospheres (10 kPa) an ideal pump is only capable of 10% of itsvolumetric pumping capacity. In practice, even this greatly reducedcapacity cannot be realized due to some small volume remaining in thedisplacement space at the end of a compression stroke in the pump. Forexample, if this remaining volume were greater than 10% of the pumpdisplacement stroke the pump would have entirely stopped pumping beforereaching the 0.1 atmosphere (10 kPa) end vacuum. Because of thisproblem, requirements for oxygen purity and production rates could notbe satisfied with an existing vacuum pump small enough to be compatiblewith portability goals if the pump had to operate at low vacuum. Theevacuate step took so much of the cycle time as it approached the lowend vacuum V that the resulting oxygen production rate became too slow.

Experiments with modifications to these simple separation steps wereperformed, trying to obtain a result that could produce 86% oxygenpurity with weaker (higher pressure) vacuum levels. The product oxygenhas a very low nitrogen partial pressure. Feeding the product oxygeninto the product end of the column during pressurize step 2 instead ofair into the feed end should lower the equilibrium pressure for nitrogenin the product end causing nitrogen to be desorbed from the product endand swept toward the feed end of the column. No nitrogen is removed fromthe column by this step because the feed end is closed. Nitrogen is justreorganized leaving less nitrogen adsorbed toward the product end andmore toward the feed end. During the feed step that follows, more of thenitrogen in the product gas stream will be adsorbed as it passes throughthe product end of the column due to the nitrogen equilibrium havingbeen set to a lower pressure by the oxygen pressurization. Higherproduct purity is obtained.

The aforementioned step was added to the test apparatus by adding thecomponents and connections in box 200 of FIG. 1. Testing proved thatoxygen could then be produced with the desired purity using an endvacuum level as high as 0.3 atmospheres (30 kPa). The column was beingevacuated to 0.3 atmospheres (30 kPa) much faster than it was able toevacuate to the original 0.1 atmospheres (10 kPa). A more practicalvacuum pump with a weaker end vacuum could now be used to power thecycle. However, significant additional time was now being spent due to alonger feed step that recycled the oxygen used to pressurize the columnback into the product oxygen accumulator. The oxygen was being passedbetween accumulator 160 and the column in steps 2 and 3 consuming extracycle time. The system increased product oxygen purity but did notproduce additional oxygen product. The effective oxygen production ratewas reduced. Further experimentation revealed that by only partiallypressurized the column to about 0.6 atmospheres (61 kPa) with productoxygen through block 200 followed by rapidly pressurizing of the columnthe rest of the way to 1 atmosphere (101 kPa) with air at the feed endthrough valve 110, the system produces the same amount of oxygen andpurity as with total pressurization by oxygen. The modified system savestime in the cycle with less pressurization oxygen to be passed betweenthe accumulator and the column.

The system with the improved version of the cycle was tested with theaddition of product oxygen fed into the product end of the column whilethe column was being evacuated through the feed end. The addition ofproduct oxygen into the feed end is done in the prior art to improveproduct purity in pressure swing (above 1 atmosphere (101 kPa)) systemsusing the Skarstrom cycle. When the improved system was tested, nobeneficial results were discerned. From the results, the following werepostulated: When a stream of air was fed through the modified adsorbentcolumn previously equilibrated at a lower partial pressure for nitrogenaccording to the level of V, a sharp dividing line called the masstransfer zone (MTZ) developed and propagated through the column in thedirection of gas flow. The MTZ was undetectable by the test setup andthe attached instrumentation. The MTZ sharply segregated the gascompositions in the column with the feed air gas composition on the feedside of the zone and the product gas composition on the product side ofthe zone. The nitrogen was being adsorbed within the narrow MTZ leavingthe gas down stream of the MTZ partially stripped of nitrogen.Unfortunately, the process could not be performed in reverse with thesame distinct segregation between the gas compositions. In the reverseprocess, oxygen rich product gas would be fed into the product end ofthe column while evacuating nitrogen rich gas out the feed end. Thereverse process did cause a mass transfer as nitrogen was desorbed fromthe column, but the mass transfer was not confined to a narrow zone. Thereverse mass transfer zone was broadly dispersed throughout the columnand immeasurable as a result of the shape and nonlinearity of thenitrogen isotherm curve compared to the oxygen isotherm (Ruthven et al.,Section 2.4.1). With no narrow zone separating the product oxygen fromthe nitrogen rich gas in the column, the oxygen is diffusing and much ofthe oxygen is swept to the feed end by the nitrogen rich gas flow and islost to the waste stream during the evacuation step. The loss of productoxygen was determined to likely be offsetting any possible gains in theamount or purity of product oxygen being separated in the tested system.

The use of this last process step was abandoned. The result was aprocess that separated product oxygen with adequate purity within arelatively small absolute pressure range of 0.3 to 1 atmosphere. Thiswould significantly reduce pumping power over that of typical pressureswing cycles operating in a higher pressure span well above 1atmosphere. By running the feed step until the mass transfer zone wasclose to breakthrough at the product end of the column, little productoxygen in the column is left to be lost during the evacuation step.These factors produced the most product oxygen volume from the feed stepfor the volume of gas pumped from the column during the evacuate step.Minimizing both the pressure range and the volume of gas to be pumpedfor a given amount of oxygen produced, in this way, required a uniquelysmall amount of pneumatic power from the pump to drive the separationsteps.

Another apparatus, shown in FIG. 3, was built to test physical scaleimplementations of the aforementioned separation cycle. In FIG. 3 thecomponents 100 through 200 are included and perform the same steps inthe same way as in FIG. 1 copied three times labeled 100A-100C through200A-200C. The cycle steps were grouped into a sequence of three equaltime intervals forming a three phase cycle. The first phase evacuatedthe column from 1 atmosphere (101 kPa) to 0.3 atmospheres (30 kPa)through the feed end. The second phase pressurized the column to 0.6atmospheres (61 kPa) with product oxygen through the product end, andthen pressurized the column to 1 atmosphere (101 kPa) with air throughthe feed end. The third phase fed air at 1 atmosphere (101 kPa) throughthe feed end and vented product oxygen from the product end. Threecolumns were connected with tubing, valves and a vacuum pump so thateach column cycled through these three phases. Each column was offset intime from the other two by one phase step. This way, at any instant intime, there was always one column being evacuated allowing the pump torun continuously allowing full use of the available pumping capacity.One column was always venting product oxygen providing a somewhatcontinuous oxygen stream. The configuration was implemented and testedfor oxygen production volume and purity with a variety of differentvalve schemes. Sequencer controlled electric solenoid valves, motordriven rotary valves, and pressure driven poppet valves in variouscombinations were tested. All tested valves worked to produce thedesired cycle steps, but there was a reduction in the purity andquantity of product oxygen produced by one of the examples. Determiningthe cause of the reduced performance in the one flawed version led tothe eventual understanding of the problem, which led to an importantimprovement in the cycle steps.

During the pressurize phase for each column, oxygen was fed into theproduct end of that column bringing the pressure to about 0.6 atmosphere(61 kPa). The oxygen was being produced by one of the other two columnsduring its feed phase offset in time to be concurrent with the columnbeing pressurized. For example, column 100A in its feed phase would passoxygen to 100B in its concurrent pressurize phase to pressurize itthrough 200A. Roughly half of the oxygen volume transferred from thecolumn in the feed phase is used to pressurize the other column. Theremaining half of the oxygen from the column in the feed phase isremoved as product by pump 151 through the check valves into theaccumulator 160. The earliest half of the oxygen stream out of the feedcolumn was used to pressurize the other column. These and the singlecolumn in FIG. 1 all performed well. However, due to a small differencein its valve scheme, the poorly performing implementation happened to beusing the last half of this oxygen stream rather than the first half topressurize the other column. The later part of the stream was slightlyless pure than the first part. This seemingly small difference wasdetermined to be the cause of the significant performance reduction ofthat implementation. Correcting this to use the first half of theproduct stream for pressurization eliminated the performance deficitthus proving the effect.

As disclosed, the system was able to beneficially increase the endpressure of the evacuate step and still maintain purity of the productby partially pressurizing the evacuated column with product oxygen. Thepartial pressure of nitrogen in the product end of the column after thephase 2 pressurization steps sets the partial pressure for nitrogen inthe beginning of the phase 3 oxygen stream due to equilibrium betweenthe gas stream and the adsorbent. Nitrogen is adsorbed in the column asthe product stream flows and some of the nitrogen is being adsorbed inthe product end. As nitrogen is adsorbed the equilibrium point fornitrogen in the product stream moves toward progressively higher partialpressures causing a gradual rise in the nitrogen content of the productstream above the initial level as the feed progresses towardbreakthrough of the mass transfer zone. Establishing a lower equilibriumlevel for nitrogen at the product end of the column therefore lowers thenitrogen level of the entire product stream due to its lower initiallevel. This is set by the nitrogen partial pressure of the lastpressurizing gas to pass through the product end of the column beingpressurized. The first part of the oxygen rich stream out of that columnduring the next feed step therefore has the least nitrogen and the lastout has the most. When used to pressurize a column, for bestperformance, the more pure first oxygen out of the feeding column shouldbe the last gas into the column being pressurized. That means theproduct gas stream should be buffered and its entire composition orderreversed before being used to pressurize a column. The configurationsthat had worked best during tests did not reverse the entire order ofthe gas stream, but at least was a step in the right direction since thesystem used the first approximate half of the product stream topressurize rather than the last half as with the configuration thatperformed poorly.

A way to implement a last in first out buffer for the product gas wasdiscovered that could be used to perform the order reversal. Anadditional column filled with LiLSX adsorbent having only a single portat one end was utilized. As gas flowed into this buffer column, oxygenwas least adsorbed and traveled the farthest down the column. Nitrogenwas strongly adsorbed and was captured by the adsorbent closest to theinlet. As gas was removed from this column, the nitrogen close to theinlet (now acting as an outlet) flowed out early due to its proximity tothe outlet and the desorbing effect of the lower nitrogen content gasflowing behind it from farther into the column. The benefit of this lastin/first out (LIFO) gas buffer was tested on the separation cycle byfirst integrating the LIFO into the earlier single column test apparatusof FIG. 1. Block 200 was removed from FIG. 1 and replaced it with block250. This includes LIFO column 251 equal in volume to separation column100. Also, valve 252 provided a controllable connection between the LIFOand the product end 102 of the separation column 100.

The test apparatus was modified to revisit and more thoroughly exploredifferent combinations of product gas feed rates, timing, and pressuresfed into the product end of the column during the evacuate step. It wasexpected that there should be some (probably small) amount of productoxygen gas that, if controlled carefully, should also help reduce thepartial pressure of nitrogen toward the product end of the column beforethe oxygen in that gas had time to diffuse to the feed end and be wastedduring evacuate. This was tested through variable flow restrictor 254with timing controlled by valve 253 linking the product end of 100 tothe LIFO 251.

The test apparatus valves were controlled to performed the followingsequence of steps:

1. Begin evacuating the column through the feed end 101.

2. At an adjustable evacuate pressure threshold, measured at 102, beginan adjustable low flow rate of product oxygen into the product end 102from the LIFO 251 through valve 253 and restrictor 254.

3. Stop evacuating and continue to flow product oxygen more rapidly byopening valve 252 from the LIFO into the product end pressurizing thecolumn to an adjustable level measured at 102.

4. Finish pressurizing the column with air through the feed end 101 toambient atmosphere.

5. Feed air into the feed end of the column 101 through 110 whilepulling oxygen out of the product end 102 to refill the LIFO 251 through252 until it is at 1 atmosphere pressure.

6. Continue feeding air and removing product oxygen from the product endof the column through 112 and pump 151 into the accumulator 160 untilthe mass transfer zone is near breakthrough.

A process to discover the best combination of pressure, flow, and timingsettings during steps 2 and 3 was performed. The best combination wasfound to include a very low oxygen flow rate into the product end of thecolumn from the LIFO during only the last part of the evacuate step. Thespecific flows and pressures are specific to the size and shape of theseparation column and need to be found experimentally for differentcolumn geometries.

The first gas out of the LIFO that fed into the product end of thecolumn toward the end of evacuate during step 2 had the highest nitrogencontent. The last gas out of the LIFO entered the column at the end ofthe pressurization during step 3 and had the lowest nitrogen content.This resulted in the column, at the end of pressurization step 4,continuously ordered with nitrogen partial pressure equilibriums highestat the feed end of the column and very low at the product end. Oxygenpartial pressures were ordered in the opposite direction so the sum ofthe two partial pressures was 1 atmosphere (101 kPa) throughout thecolumn. With this, oxygen purity 86% and higher were obtained with endvacuums as high as 0.5 atmospheres (51 kPa). This is a vacuum swing ofonly 0.5 atmospheres (51 kPa) compared to a pressure swing of 2atmospheres (203 kPa) for the typical Skarstrom PSA cycle. A verysignificant reduction in pneumatic power resulted. Also, the relativelyweak vacuums can be achieved rapidly by a smaller and more practicalvacuum pump thus shortening the evacuate step and the total cycle timefor a given amount of oxygen produced.

Using these separation cycle steps, the design shown in FIG. 4 wasconstructed for a portable oxygen concentrator. The portable system isconfigured with two separation columns 400 and 401 to allow someoverlapping of concurrent cycle steps, and one LIFO buffer 402 that wasshared during the cycle steps occurring in the two columns.

In FIG. 4 cycle step 1 is first performed on column 400. With valves 410and 411 positioned as illustrated, variable rate vacuum pump 420evacuates column 400 from the feed end pulling gas through valves 410and 411 then passing the gas from pump 420 back through valve 411 asshown and out port 430 to atmosphere. Valves 413, 415, and 417 areclosed.

Step 2 begins on column 400 when the vacuum level in column 400 reachesa selected threshold measured at monitoring point 440. Valve with flowrestrictor 415 is opened to establish a controlled flow of oxygen richgas from the LIFO 402 into the product end of column 400 while theevacuation of column 400 that began in step 1 continues. Step 2 is endedafter a selected amount of time.

Step 3 begins on column 400 with valve 411 switched to the oppositestate halting evacuate of column 400. Valve 413 is opened allowing amore rapid flow of oxygen from the LIFO 402 into column 400 pressurizingit to about 0.6 atmospheres (61 kPa).

Step 4 begins with valve 410 switched to the opposite state providing apath for ambient air through port 430, check valve 412 and valve 410into the feed end of column 400 pressurizing it to 1 atmosphere (101kPa). Valves 413, 415, and 417 are all closed during step 4.

Step 5 refills LIFO 402, which was left at a partial vacuum aftertransferring gas to pressurize column 400 in step 3. Valve 413 is openeddrawing oxygen rich gas from column 400 into LIFO 402 until it reaches 1atmosphere (101 kPa). Column 400 remains at 1 atmosphere (101 kPa)throughout this step with the path to its feed end still open to ambientair as in step 4. All activity is stopped after step 5 waiting for apatient with a nose cannula (not illustrated) connected to port 431 tobegin a breath inhalation.

Step 6 begins when the patient inhales with valves 413 and 415 closedand 410 and 411 in the opposite state shown. The vacuum pump 420 startsimmediately drawing air from ambient at port 430, through valve 411 andinto the inlet of pump 420. That air is pumped through vacuum pump 420and back through the other path in valve 411, through valve 410 and intothe feed end of column 400. The pressure in column 400 is slightlyraised in this way until it is sufficient to open check valve 417 andvent product oxygen out of port 431 to the nose cannula worn by thepatient. Vacuum pump 420 is controlled in its flow rate and pumpingduration to deliver a pulse of oxygen rich gas to the patient that isshaped in its flow rate and duration.

The steps outlined above are performed on column 400 or on 401 selectedby the state of valve 410. This toggles from one column to the otherevery patient inhalation. Some cycle steps are overlapped and performedconcurrently on the two columns to save cycle time and to be able tokeep up with a rapid breathing rate.

The portable oxygen concentrator using this separation apparatusdelivers oxygen to a patient in pulses that are synchronized with thebeginning of their breath inhalation. This eliminates the waste ofoxygen and associated energy that occurs during a patient's exhalationor breath hold. The separation cycle is well suited to that requirement.In one embodiment, the cycle is stopped and held at the end of eachpressurization step until the patient's inhalation began. The secondhalf of the feed step is then rapidly completed producing product oxygenwithin about 0.4 seconds with a small pressure rise above 1 atmosphere(101 kPa), just enough to pressurize the product oxygen to induce flowthrough a short breathing cannula. While the patient continues throughthe breathing cycle sequence of inhalation, exhalation, and breath hold,the next separation cycle steps are being performed, throughpressurization, and again held waiting for the next inhalation. There isno need for any added oxygen storage or dispensing components. Oxygen isseparated and dispensed directly from the separation column, in realtime, through the cannula to the patient. All activity involved in theseparation cycle, including operation of the pump, is halted whilewaiting for the next inhalation. This provides additional energysavings, important in the portable application.

The oxygen flow pulses need to be variable in volume to match thespecific needs of each patient. This separation cycle is suited to thatrequirement. The amount of oxygen produced in one feed step of the cycleis variable in accordance with the end vacuum level. Lower end evacuatepressures produced larger volumes of oxygen during each feed. Higher endvacuums produced smaller volumes while the oxygen pressurization of thecolumns from the LIFO buffer maintained adequate oxygen purity by alwayskeeping the product end of the columns at a low nitrogen partialpressure. Higher end evacuate pressures require less pump energy andallowed faster evacuations and a greater fraction of time spent with thecycle on hold. This produces energy savings when reduced oxygen volumesare selected.

Minimizing the work load on the pump in these ways reduces the peakpumping requirements and therefore generally reduces the size and weightof the vacuum pump. Using an adsorbent with a steep nitrogen isothermand cycling it below 1 atmosphere where it is the steepest allowsadsorbent columns with less size and weight for a given nitrogencapacity. Smaller than normal amounts of oxygen are lost to the wastestream during the evacuate step due in combination to the low oxygenadsorption capacity of the adsorbent in the vacuum range, the cycle thatpasses little oxygen from the product end to the waste stream of thecolumn during evacuate, and little oxygen left in the column at the endof feed due to operation near breakthrough of the mass transfer zone.This all contributes to a high oxygen recovery rate for the cycle andtherefore less waste stream flow created for a given oxygen productflow. Smaller scale pump, valves, and connecting components can be usedto handle this reduced evacuated waste stream.

A portable medical oxygen concentrator using this separation apparatuswith a custom designed high efficiency vacuum pump separates oxygenwhile consuming 12.7 watts per liter per minute of 88% pure oxygen. Theportable concentrator weighs 2.5 pounds per liter per minute fullypackaged. For comparison, the Invacare concentrator that was tested andutilizes the Skarstrom pressure swing cycle consumed 70 watts per literper minute and weighs 10.4 pounds per liter per minute.

While the invention has been described with reference to an exemplaryembodiment(s), it will be understood by those skilled in the art thatvarious changes may be made and equivalents may be substituted forelements thereof without departing from the scope of the invention. Inaddition, many modifications may be made to adapt a particular situationor material to the teachings of the invention without departing from theessential scope thereof. Therefore, it is intended that the inventionnot be limited to the particular embodiment(s) disclosed, but that theinvention will include all embodiments falling within the scope of theappended claims.

1. An apparatus for separation of gases from ambient air, the apparatuscomprising: at least one separation column, the separation columncontaining an inlet at a first end and an outlet at a second end,wherein the column comprises an adsorbent having a high capacity fornitrogen adsorption; a buffer column having a single inlet at a firstend, wherein the buffer column comprises an adsorbent having a highcapacity for nitrogen adsorption; and a valve system that connects theseparation column and buffer column.
 2. The apparatus of claim 1 furthercomprising a vacuum pump.
 3. The apparatus of claim 1 wherein theadsorbent comprises an isotherm defining the nitrogen capacity as afunction of nitrogen partial pressure resulting in a substantiallycurved lined starting with a steep slope at 0 atm that greatly decreasesas pressure increases to about 3 atm.